Making the survey123 public

When I choose "Everyone(public)" as who can submit data to this survey, does it means that everyone in the world can search this survey and access it? or it means that everyone who has the link can open and fill it? If it can be accessed by everyone in the world, can we have a security code for the participants to prevent non targeted individuals to access the survey?

While sharing with everyone does allow anyone to submit to the survey, a person would need to search ArcGIS Online in order to find it.

It is possible to include a hidden question to store a validation value in a survey, and then include the value as part of the link sent to people (see Understanding Survey123's custom URL Scheme for providing values as part of a link); if you set the other questions in the survey to not relevant unless the code was present, that would hide the form otherwise.